I've been a long-time user of the WallF BaseNEncodings.Net library. It's the only library I'm aware of that was explicitly built to RFC 4648. I submitted a pull request to retarget the library to netstandard1.1, but received no response from the author. While I had hoped the project were still maintained, that seems not to be the case. To that end, I have forked the library and republished it. The new repo is https://github.com/SapientGuardian/BaseNEncodings.Net. Happy Encoding!